The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are illustrated in FIG. 1. They are lower magnetic shield 10 on which is seed layer 11. Antiferromagnetic (AFM) layer 12 is on seed layer 11. Its purpose is to act as a pinning agent for a magnetically pinned layer. The latter is typically a synthetic antiferromagnet formed by sandwiching antiferromagnetic coupling layer 14 between two antiparallel ferromagnetic layers 13 (AP2) and 15 (AP1).
Next is a copper spacer layer 16 on which is low coercivity (free) ferromagnetic layer 17. Capping layer 18 lies atop free layer 17. When free layer 17 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 10-20%.
Earlier GMR devices were designed to measure the resistance of the free layer for current flowing parallel to its two surfaces. However, as the quest for ever greater densities has progressed, devices that measure current flowing perpendicular to the plane (CPP) have also emerged.
Another consideration is the R.A product. This is the resistance of the CPP device times its cross-sectional area. Normally, the transverse resistance of a layer increases as its cross-sectional area decreases so the R.A product tends to be a constant. However, by arranging for the current path to be confined to only certain parts of the free layer, a larger value for R.A can be achieved. Devices of this type are referred to as CCP (confined current path) devices.
If copper spacer layer 16 in FIG. 1 is replaced by a very thin layer of insulating material, a magnetic tunneling junction (MTJ) device that depends on the TMR (tunneling magneto resistance) effect is formed. In this device, the tunneling current that passes through layer 16 (when it is an insulator) from layer 15 to layer 17, is measurably larger when the directions of magnetization of layers 15 and 17 are parallel (as opposed to antiparallel).
Typically, in both TMR and CPP-GMR head structures, a bottom synthetic spin valve type film stack has been employed for biasing reasons, together with a Ta—NiCr based seed layer for the AFM and a CoFe/NiFe composite free layer.
In a typical TMR or CPP spin valve structure of any case shown above, the conventional seed layer thickness of Ta/NiCr is about 70 Å, and the AP1 or AP2 thickness is in the range of 20-50 Å, and the free layer thickness is in the range of 30-60 Å. For ultra-high density read head applications, a thinner total film stack thickness is preferred in order to achieve higher resolution. Since reducing the magnetic layer thickness also reduces the MR ratio of TMR or CPP, it would be preferable to reduce the thickness of the nonmagnetic layers. The present invention provides a solution to this problem.
A routine search of the prior art was performed with the following references of interest being found:
U.S. Patent Application 2004/0179311 (Lin et al) Headway states that Ta and NiCr form a typical seed layer. U.S. Pat. No. 6,936,903 (Anthony et al) and U.S. Pat. No. 6,919,594 (Sharma et al) both make reference to seed layers of Ta/Ru. but neither invention gives details concerning their thicknesses. Nor do they specify that these may be used with a pinning layer of IrMn of the thickness taught by the present invention.
U.S. Patent Application 2005/0164414 (Desk) teaches that the seed layer may be NiFe, Cu, or other materials and have a thickness of 5 to 50 Angstroms. U.S. Patent Application 2005/0174702 (Gill) describes a seed layer comprising 30 Angstroms of Ta and 20 Angstroms of NiFeCr. U.S. Pat. No. 6,903,904 (Li et al) Headway discloses a seed layer of Ta/NiFe of 30-70 Angstroms.
U.S. Pat. No. 6,806,804 (Saito et al) teaches a 30 Angstrom NiFe seed layer over a Ta underlayer. U.S. Pat. No. 6,862,159 (Hasegawa) shows a Ta/Cr or Ta/NiFe seed layer where the Cr or NiFe layer is 30 Angstroms thick. Finally, U.S. Pat. No. 6,862,158 (Hasegawa et al) teaches a Ta/Cr seed layer formed using Ar gas wherein the Cr layer is 15-50 Angstroms. It is stated that under 15 Angstroms, the Cr layer is not uniform.